1. Field of the Invention
The present invention relates to the configuration of a BPSK (Binary Phase Shift Keying) modulator and a mixer which are used in microwave communications equipment and a method for adjusting them.
2. Description of the Prior Art
Microwave phase modulators of the prior art are constructed so as to produce phase-modulated waves by switching between signals in opposite phases which are generated with a .lambda./2 (.lambda. is the wavelength of the center frequency of the modulated wave) line, two diodes, etc.
FIG. 1 is a view illustrative of the configuration of such a prior art modulator.
In this drawing, a carrier microwave input line 18 and a semi-circle line 19 (a length of .lambda./2) are laid on a dielectric substrate 30. Connected to the midpoint of the .lambda./2 line 19 is a line 20 for applying a modulating signal (baseband signal).
Since the line 19 is .lambda./2 long, the signal has opposite phases at the input end and at the output end. In addition, diodes 21, 22 are placed in series connecting the two ends of the line 19, with an output line 23 extending from the point of connection between the diodes 21, 22.
These lines 19, 20, 23 are striplines on the substrate 30. In cases where the modulating signal applied to the line 20 is a pulse signal which causes switching with the diodes 21, 22, the signals in opposite phases which have been generated at the input and output ends are alternately output by switching with the diodes. As a result, BPSK (Binary Phase Shift Keyed) waves modulated to the 0 phase or the .pi. phase are output via the output line 23.
In cases where the signal input via the line 20 is at a signal level which allows operation of the diodes in a nonlinear characteristic region, that type of modulator of the prior art also serves as a mixer (frequency converter circuit) which provides output signals as balanced modulated signals at the sum frequency and the difference frequency of the two signals input via the line 18 and the line 20.
Such configuration is described in, for example, Japanese Published Unexamined Patent Application No.3-92341 (1991).
Although the above-described phase modulator of the prior art is indeed a microwave converter circuit in theory, in actuality it has a problem in that it is hard to mount the minute diodes on the dielectric substrate. There is an additional problem in that deviation from the ideal characteristics of the 0 phase and the .pi. phase are caused by asymmetry of the line patterns, variations in the patterns due to errors during manufacture, variations in characteristics of the diodes, etc. In addition, it is necessary to retrofit the modulator with a capacitive stub, for example, for phase adjustment in order to eliminate such deviation, and this complicated and difficult adjusting process presents another drawback.
More specifically, the material for the capacitive stub used for the adjustment is usually a piece of a metal such as iron or indium which is repeatedly soldered to or desoldered from an appropriate point for adjustment of the phase-modulating characteristics. This process also has a problem in that the size and the point of mounting of the metal piece and the amount of the solder have delicate effects on the characteristics, there are no guides for the point of attachment and the capacity of the capacitive stub, and therefore the process has an extremely poor working efficiency due to the poorly reproducible characteristics.